This section provides background information related to the present disclosure which is not necessarily prior art. This section also provides a general summary of the disclosure, and is not a comprehensive disclosure of its full scope or all of its features.
Gas chromatography (GC) is a common, powerful method for measuring quantities of specific substances in a sample. Photoionization detectors (PIDs) are being widely used in GC systems due to their high sensitivity and large dynamic range. In a typical design, a PID consists of a vacuum UV (VUV) lamp filled with low-pressure rare gases such as xenon, krypton, and argon, which generate photons ranging from 9.6 eV to 11.8 eV, and a sealing window made of a UV transparent crystal (e.g., LiF, MgF2, and CaF2). However, those PIDs suffer from a very limited lifetime of a few hundred to a couple of thousand hours and gradual performance degradation due to gas leakage and window degradation caused by crystal solarization, water etching, and contamination of chemical compounds. Thus, constant maintenance and calibration are needed during their entire operation lifetime. Furthermore, although those PIDs are able to provide sufficient photon energy (e.g., 11.8 eV by argon based PIDs with a LiF window) to ionize most of chemical compounds, there still exist many important compounds with the ionization potential (IP) above or close to 11.8 eV, such as Freon (IP: 11.77 eV-12.91 eV), methane (IP: 12.98 eV), chlorine trifluoride (IP: 12.65 eV), dichlorofluoromethane (IP: 12.39 eV), phosgene (IP: 11.77 eV), and ethane (IP: 11.65 eV), just to name a few, which cannot be analyzed with those PIDs.
Atmospheric pressure rare gas discharge based PIDs usually have a windowless design, which maximizes UV transmission and can potentially extend the PID lifetime. Helium is typically used in this type of PID. Known as Hopfield emission, which results from the transition from the diatomic helium state to the dissociative helium state, photons ranging from 13.5 eV to 17.5 eV can be generated during the helium discharge process, making the helium discharge PID (HD-PID) virtually a universal detector for gas analysis. In an HD-PID, helium plasma is usually generated by direct current (DC) discharge, pulsed discharge, or dielectric barrier discharge (DBD). DC discharge relies on high voltage (or electric field) to break down helium into positive ions and electrons, thus generating gas plasma. Recently, a miniaturized HD-PID based on DC discharge was demonstrated with 550 VDC across a 20 μm gap and power consumption of only 1.4 mW. A detection limit on the order of 10 pg was achieved. Pulsed discharge is also called pulsed DC discharge. The operation principle is similar to DC discharge, but uses 1 kHz-1 MHz pulses to lower average power consumption and achieve better and more stable ionization/excitation. The popular PIDs from Valco Instruments are based on the pulsed discharge technology. Recently, a micro-pulsed discharge PID was also developed with the size as large as 10 cm3 and a detection limit of a few pg. However, one of the major drawbacks of DC discharge is the sputtering effect, i.e., high-speed ions continuously bombard the cathode material, which limits the lifetime of the detector and requires constant maintenance of electrodes and chambers (such as electrode replacement and discharge chamber cleaning). For pulsed discharge, since the duty cycle and hence the average electrical power are lower, the overall sputtering effect is lower. However, due to high instantaneous power, the sputtering cannot be completely prevented.
DBD uses high-voltage (1-100 kV) high-frequency (up to a few MHz) potential to generate atmospheric pressure plasma. In a DBD design, dielectric materials (e.g., glass, polymer, and quartz, etc.) are placed on the electrode surface facing the discharge chamber, thus forming a protection layer. Compared to the two aforementioned discharge methods, the DBD method is advantageous in a homogenous discharge and very long electrode operation lifetime. Therefore, it has become the preferred method for atmospheric pressure plasma generation and been applied in numerous applications. Recently, a few DBD based helium discharge PIDs became commercially available (such as BID-2010 Plus from Shimadzu and DBDID from ABB Inc.) with the detection limit ranging from a few tens of picogram to sub-picogram. However, the existing DBD based HD-PIDs (such as BID-2010 Plus and DBDID) are bulky (similar to the dimensions and weight of a commercial FID) and power intensive (DBDID: 12 W) and require a large auxiliary helium flow rate (50-100 mL/min) and long warm-up time. While those HD-PIDs can be used in benchtop GC systems, they are not suitable for portable or micro-GC systems for field applications. Accordingly, advanced DBD based HD-PIDs are desired that address these shortcomings.
Further areas of applicability will become apparent from the description provided herein. The description and specific examples in this summary are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.